Synthesis and reactivity of molybdenum imido alkylidene bis-pyrazolide complexes

Article abstract of DOI:10.1039/c0dt00315h

Reaction of Li(3,5-R₂-pyrazolide) (R = tBu or Ph, dXpz) with Mo(NAr)(CHCMe₂Ph)(OTf)₂(DME) yields Mo(NAr)(CHCMe ₂Ph)(dXpz)₂ in good yield. These complexes react with alcohols or the surface silanols of silica, to yield bis-alkoxy and surface mono-siloxy alkene metathesis catalysts, respectively.

Full text of DOI:10.1039/c0dt00315h

View Article Online / Journal Homepage / Table of Contents for this issue
This article is published as part of the Dalton Transactions themed issue entitled:
Bridging the gap in catalysis via
multidisciplinary approaches
Guest Editors: Christophe Coperet and Rutger van Santen
Université de Lyon, France and Eindhoven University of Technology, The Netherlands
Published in issue 36, 2010 of Dalton Transactions
Image reproduced with the permission of Dieter Vogt
Articles in the issue include:
Molecular understanding of alkyne hydrogenation for the design of selective catalysts
Javier Pérez-Ramírez, Blaise Bridier and Nuria Lopez
Dalton Trans., 2010, DOI: 10.1039/C0DT00010H
Molecular weight enlargement–a molecular approach to continuous homogeneous catalysis
Michèle Janssen, Christian Müller and Dieter Vogt, Dalton Trans., 2010,
DOI: 10.1039/C0DT00175A
Structure Determination of Zeolites and Ordered Mesoporous Materials by Electron
Crystallography
Xiaodong Zou, Junliang Sun, Dalton Trans., 2010, DOI: 10.1039/C0DT00666A
Metal-Catalyzed Immortal Ring-Opening Polymerization of Lactones, Lactides and Cyclic
Carbonates
Noureddine Ajellal, Jean-François Carpentier, Clémence Guillaume, Sophie M. Guillaume, Marion
Helou, Valentin Poirier, Yann Sarazin and Alexander Trifonov, Dalton Trans., 2010, DOI:
10.1039/C001226B
Visit the Dalton Transactions website for more cutting-edge inorganic and organometallic research
www.rsc.org/dalton

PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis and reactivity of molybdenum imido alkylidene bis-pyrazolide
complexes†
David Gajan,a,b Nuria Rendo´n,a Keith M. Wampler,c Jean-Marie Basset,a Christophe Cope´ret,*a Anne Lesage,b
Lyndon Emsleyb and Richard R. Schrock*c
Received 14th April 2010, Accepted 8th June 2010
DOI: 10.1039/c0dt00315h
Reaction of Li(3,5-R2-pyrazolide) (R = tBu or Ph, dXpz) with Mo(NAr)(CHCMe2Ph)(OTf)2(DME)
yields Mo(NAr)(CHCMe2Ph)(dXpz)2 in good yield. These complexes react with alcohols or the surface
silanols of silica, to yield bis-alkoxy and surface mono-siloxy alkene metathesis catalysts, respectively.
other materials were prepared according to published procedures
1
Introduction
cited in the text or obtained from commercial sources and purified
by standard procedures. Silica (Aerosil Degussa, 200 m2 g-1) was
compacted with distilled water, dried, calcined at 500 ◦C under air
for 2 h and t◦reated under vacuum (1.34 Pa) at 500 ◦C for 12 h and
then at 700 C for 4 h (support referred to as SiO2-(700)). Pentane
and toluene were distilled from NaK and sodium benzophenone
ketyl under N2, respectively. Propene (Scott, 99.95%) was purified
Molybdenum imido alkylidene diphenylamido1 and pyrrolyl2
complexes have served as precursors to prepare highly ac-
tive homogeneous3 and heterogeneous4 alkene metathesis Mo-
based catalysts. The key to high reactivity is the formation of
(Y)(X)Mo(NAr)( CHR) with an asymmetric metal center (X π
Y in terms of s-donating ability).5 Considering the unprecedented
high performances of monoaryloxide monopyrrolyl (MAP)3 cata-
lysts as well as the large influence of the pendant X ligand for silica
supported systems, ( SiO)(X)Mo(NAr)( CHR),4 we decided
to explore the formation of the corresponding pyrazolide family
˚
over R3-11 BASF catalyst/MS 4 A prior to use.
Measurements. NMR spectra were recorded at 298 K on Var-
ian INOVA spectrometers operating at 500 MHz (1H). Chemical
shifts for 1H and 13C spectra were referenced to the residual
1H/13C resonances of the deuterated solvent and are reported
as parts per million relative to tetramethylsilane. Elemental
analyses were performed by Midwest Microlabs, Indianapolis,
IN. Elemental analyses for surface complexes were performed
at the Mikroanalytisches Labor Pascher. Infrared spectra were
recorded on a Nicolet 550-FT by using an infrared cell equipped
with CaF2 windows, allowing in situ studies. Typically, 16 scans
were accumulated for each spectrum (resolution, 2 cm-1). Liquid
phase analyses were performed on a Hewlett Packard 5890 series II
GC apparatus equipped with a FID detector and a Fame column
(50 m X 0.25 mm). Products were identified by GC-MS. All solid
state NMR spectra were recorded under MAS on a Bruker Avance
500 spectrometer with a conventional double resonance 4 mm
CP-MAS probe. The MAS frequency was set to 10 kHz for the
experiments reported here. The samples were introduced in a 4 mm
zirconia rotor in the glove box and tightly closed. Proton and
carbon chemical shifts are reported in ppm downfield from liquid
SiMe4 (0 ppm).
5
because these ligands can assume a variety of binding modes: h ,
2
1
2
h , h , or m , which could affect the overall performance of the
catalyst.6
2
Experimental
General procedure
General. All manipulations were performed in oven-dried
(150 ◦C) glassware under an atmosphere of nitrogen on a Schlenk
line or in a Vacuum Atmospheres HE-492 drybox. For synthesis
and treatment of the surface species, reactions were carried
out using high vacuum lines (1.34 Pa) and glovebox techniques
(Argon).
Materials. HPLC grade organic solvents were sparged with
nitrogen, passed through activated alumina, and stored over
˚
4 A Linde-type molecular sieves prior to use. Benzene-d6 was
dried over sodium/benzophenone ketyl and distilled in vacuo
prior to use. Mo(NAr)(CHCMe2Ph)(OTf)2(DME),7 Li(3,5-di-
t-butylpyrazolide) {Li(dtpz)},8 and Li(3,5-diphenylpyrazolide)
Crystallography. Low temperature diffraction data were col-
lected on a Siemens Platform three-circle diffractometer coupled
to a Bruker-AXS Smart Apex CCD detector with graphite-
9
{Li(dppz)} were prepared according to literature procedures. All
aUniversite´ de Lyon, Institut de Chimie de Lyon, C2P2 UMR 5365 (CNRS –
CPE –UCBL), 43 Bd du 11 Novembre 1918, F-69616 Villeurbanne Cedex,
France. E-mail: coperet@cpe.fr; Fax: 33 (0)472431795; Tel: 33 (0)47243
2645
˚
monochromated Mo-Ka radiation (l 0.71073 A), performing j-
and w-scans. All structures were solved by direct methods using
SHELXS10 and refined against F2 on all data by full-matrix least
squares with SHELXL-97.11 All non-hydrogen atoms were refined
anisotropically. Unless described otherwise, all hydrogen atoms
were included in the model at geometrically calculated positions
and refined using a riding model. The isotropic displacement
parameters of all hydrogen atoms were fixed to 1.2 times the U
value of the atoms they are linked to (1.5 times for methyl groups).
bUniversite´ de Lyon, CNRS/ENS Lyon/UCB Lyon 1, Centre de RMN a`
Tre`s Hauts Champs, 5 rue de la Doua, 69100 Villeurbanne, France
cDepartment of Chemistry, Massachusetts Institute of Technology, 77
Massachusetts Ave., Room 6-331, Cambridge 02139, MA, USA.
E-mail: rrs@mit.edu
† Electronic supplementary information (ESI) available: Fig. S1–S9. See
DOI: 10.1039/c0dt00315h
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 8547–8551 | 8547
©

Synthesis of molecular complexes
Grafting molecular complexes onto silica
Mo(NAr)(CHCMe2Ph)(dppz)2 (1-dppz-Ph). The complex
Mo(NAr)(CHCMe2Ph)(OTf)2(DME) (1.00 g, 1.28 ol) was
weighed and dissolved in 10 mL of Et2O. Li(dppz) (0.586 g,
2.65 mmol, 2.05 equiv.) was added as a solid to the rapidly stirring
solution kept at -30 ◦C over the course of 5 min. The reaction
mixture was then allowed to warm to room temperature and was
stirred for 90 min during which time it became translucent yellow
and a pale yellow precipitate formed. All volatile components
were removed in vacuo and the resulting yellow powder was
extracted with 20 mL of toluene which was filtered through
medium porosity sintered glass frit. The toluene was removed
in vacuo then the resulting solid was triturated in 20 mL of
pentane. Mo(NAr)(CHCMe2Ph)(dppz)2 (0.975 g, 1.15 mmol,
90%) was isolated by filtration as a pale yellow microcrystalline
solid. X-Ray quality crystals were grown from a mixture
of pentane and toluene at -30 ◦C. The 13C labelled analog,
Mo(NAr)(13CHCMe3)(dppz)2 was prepared similarly utilizing
Grafting of [Mo(NAr)( CHCMe2Ph)(dppz)2] (1-dppz-Ph) on
SiO2-(700) monitored by in situ IR spectroscopy. Silica (30 mg)
was pressed into a 18 mm self-supporting disk, put into a sealed
glass high vacuum reactor equipped with CaF2 windows. After
calcination at 500 ◦C under air for 2 h, the silica disk was treated
under vacuum (1.34 Pa) at 500 ◦C for 12 h and then at 700 ◦C
for 4 h. The silica support thus obtained, referred to as SiO2-(700)
,
(30 mg, 78 mmol SiOH), was then immersed into a solution of
[Mo(NAr)( CHCMe2Ph)(dppz)2] (7 mg, 86 mmol, 1.1 equiv.)
in benzene (10 mL) at 25 ◦C. After 3 h, the disk was washed
three times with benzene (3 ¥ 10 ) and then dried under high
vacuum (1.34 Pa) at 25 ◦C for 1 h. IR: 3745, 3618, 3068, 3032,
2965, 2930, 2872, 1566, 1495, 1469, 1445, 1425, 1388, 1364 cm-1.
Grafting of [Mo(NAr)( CHCMe2Ph)(dppz)2] (1-ddpz-Ph) on
SiO2-(700) by impregnation (1-dppz-Ph/SiO2). Preparation of
[[( SiO)Mo(NAr)( CHCMe2Ph)(dppz)] (2-dppz-Ph). A mix-
ture of 1-dppz-Ph (84 mg, 0.1 ol) and SiO2-(700) (350 mg, 0.091 mmol
SiOH) in benzene (8 ) was stirred at 25 ◦C for 3 h. After filtration,
the yellow solid was washed three times with benzene (3 ¥ ca. 5 )
and filtered. The resulting yellow powder was dried under high
vacuum (1.34 Pa) at 25 ◦C for 1 h to yield a yellow solid. All
the filtrate solutions were collected and distilled off. The resulting
1
Mo(NAr)(13CHCMe3)(OTf)2(DME). H NMR (C6D6): d 13.10
(s, 1 H, JCH 124 Hz, Mo CH), 7.71 (d, 8 H, dppz ortho), 7.28
(d, 2 H), 7.14 (app. t, 8 H, dppz meta), 7.06 (t, 4 H, para), 7.03
(s, 2 H, dppz), 6.93 (m, 6 H, NAr + CMe2Ph), 3.74 (septet, 2 H,
1
CHMe2), 1.47 (s, 6 H, CMe2Ph), 0.98 (d, 12 H, CHMe2). 13C{ H}
NMR (C6D6): d 305.4(Mo C), 153.1, 152.3, 149.2, 148.0, 133.8,
129.2, 129.0, 128.6, 128.4, 127.5, 126.7, 126.6, 123.6, 107.6, 55.7,
30.9, 28.8, 24.0. Anal. calcd for MoC52H51N5Mo: C 74.18, H 6.11,
N 8.32; found: C 74.20, H 6.18, N 8.31.
1
residue was analyzed by quantitative H NMR spectroscopy (in
C6D6) using Cp2Fe (16 mg, 86 mmol, 10 H) as internal standard:
9.5 mmol of dppz (0.11 H for the NH proton) was formed during
grafting, that is 0.2 equiv. of dppz/Mosurf. 1H MAS NMR: d 12.7,
7.8, 4.4, 1.8 ppm. 13C CP MAS NMR: d 152, 149, 128, 108, 54,
28, 22 ppm. Elemental analysis: 1.62 0.05%wt Mo.
Mo(NAr)(CHCMe2Ph)(dtpz)2 (1-dtpz-Ph). The complex
Mo(NAr)(CHCMe2Ph)(OTf)2(DME) (1.00 g, 1.28 ol) was
weighed and dissolved in 10 mL of Et2O. Li(dtpz) (0.482 g,
2.65 mmol, 2.05 equiv.) was added as a solid to the rapidly stirring
solution kept at -30 ◦C over the course of 5 min. The solution
was then allowed to warm to room temperature and was stirred
for 90 min during which time the color of the solution became
dark yellow. All volatile components were removed in vacuo and
the resulting powder was extracted with 20 mL of pentane, which
was filtered through a medium porosity sintered glass frit. The
solution was then concentrated in vacuo and then left for 18 h at
-30 ◦C. Mo(NAr)(CHCMe2Ph)(dtpz)2 (0.717 g, 0.94 mmol, 73%)
was isolated as yellow crystalline solid. 1H NMR (C6D6): d 13.35
(s, 1 H, JCH 124 Hz, Mo CH), 7.76 (d, 2 H, JCH 8.2 H, ind),
7.31 (d, 2 H, NAr), 7.09 (t, 2 H, CMe2Ph), 7.02 (m, 4 H, NAr +
CMe2Ph), 6.19 (s, 2 H, dtpz), 4.08 (septet, 2 H, CHMe2), 1.81
(s, 6 H, CMe2Ph), 1.31 (s, 36 H, t-Bu), 1.12 (d, 12 H, CHMe2).
Grafting of [Mo(NAr)( *CHCMe3)(dppz)2] (1*-dppz-Me)
onto SiO2-(700) by impregnation. Preparation of [( SiO)Mo-
(NAr)( *CHCMe3)(dppz)] (2*-dppz-Me). This reaction was
carried out as described above by using 1*-dppz-Me in place of
1-dppz-Ph.
Grafting of [Mo(NAr)( CHCMe2Ph)(dtpz)2] (1-dtpz-Ph)
onto SiO2-(700) by impregnation (1b-Ph/SiO2). Preparation of
[( SiO)Mo(NAr)( CHCMe2Ph)(dtpz)] (2-dtpz-Ph). The com-
plex 1-dtpz-Ph (115 mg, 0.14 ol) was grafted on SiO2-(700) (504 mg,
0.134 mmol SiOH) using the procedure described above for 1-dppz-
Ph. Analysis of the filtrate showed the formation of 27.4 mmol of
dtpz (0.3 H for the NH proton) during grafting, that is 0.3 equiv.
1
of dtpz/Mosurf. H MAS NMR: d 12.7, 6.9, 3.6, 2, 1 ppm. 13C
CP MAS NMR: d 162, 150, 128, 106, 50, 30, 23 ppm. DRIFT:
3608, 3064, 3030, 2967, 2932, 2910, 2874, 1601, 1560, 1521, 1495,
1486 cm-1. Elemental analysis: 1.58 0.05%wt Mo.
1
13C{ H} NMR (C6D6): d 303.5 (Mo C), 159.3, 152.8, 150.8,
148.3, 128.7, 126.8, 123.8, 104.1, 55.8, 32.5, 31.3, 28.7, 24.8. Anal.
calcd for MoC44H67N5: C 69.36, H 8.86, N 9.19; found: C 69.27,
H 8.88, N 9.14.
Metathesis
General method employed for metathesis reactions in the solution
phase. For metathesis reactions of diallyltosylamine or allyl
ether, a 0.2 M solution of substrate in C6D6 and 10 mL of anisole
(as internal standard) were placed in a Teflon capped NMR tube
and the noted amount of catalyst was then added. The tube was
capped and the solution was allowed to stand at room temperature.
Conversion was determined via 1H NMR spectroscopy.
General procedure for in situ reactions with alcohols and phenol.
The appropriate bispyrazolide complex was dissolved in C6D6
containing 1.1 mg ferrocene as internal standard. An alcohol or
phenol was then added to the stirred mixture in one portion as a
solid and the solution was stirred for 15 min. The entire reaction
1
mixture was transferred to a J-Young tube and the H NMR
General method employed for the metathesis of propene in the
gas phase catalyzed by 1-Ph/SiO2 performed in a flow reactor.
spectrum recorded.
8548 | Dalton Trans., 2010, 39, 8547–8551
This journal is
The Royal Society of Chemistry 2010
©

Table 1 Crystal data and structure refinement of 1-dppz-Ph
Representative procedure. The solid 1-dppz-Ph/SiO2 (105 mg,
17.5 mmol) was loaded in a flow reactor in the glove-box, the
isolated reaction chamber was then connected to the propene line,
the propene pressure was set to 1 bar, and the tubes were flushed
with propene for 2 h. The flow rate was set to 400 mL ¥ min-1
(16 (propene) ¥ mol(Mo)-1 ¥ s-1), the temperature was set to 30
◦C and the opening of the valve corresponds to the beginning of
the catalysis. The reaction was monitored by GC using an auto-
sampler.
Empirical formula
Formula weight
Temperature
MoC52H51N5
841.92
100(2) K
˚
Wavelength
0.71073 A
Crystal system
Space group
Unit cell dimensions
Monoclinic
P21/n
˚
a 13.0553(17) A
˚
˚
b 19.823(3) A
c 16.594(2) A
a 90◦
b 100.415(2)◦
g 90◦
3
Results and discussions
3
˚
V
Z
4223.6(9) A
4
Synthesis and structures of the molecular bis-pyrazolide complexes
Dcalcd
1.324 Mg m-3
0.353 mm-1
1760
The reaction of 2 equiv. of the bulky Li pyrazolides, Li(3,5-di-
t-butylpyrazolide), Li(dtpz), or Li(3,5-diphenylpyrazolide),
m
F(000)
Crystal size
0.50 ¥ 0.50 ¥ 0.50 mm3
1.62–27.48◦
16 £ h £ 16
-25 £ k £ 23
-21 £ l £ 21
71 070
Li(dppz),
with
Mo(NAr)(CHCMe2Ph)(OTf)2(DME)
at
-30 ◦C in Et2O gives the corresponding bis-pyrazolide Mo
complexes Mo(NAr)(CHCMe2Ph)(dppz)2 (1-dppz-Ph) and
Mo(NAr)(CHCMe2Ph)(dtpz)2 (1-dtpz-Ph) in 73 and 90% yield,
respectively (Scheme 1). 1H NMR spectra at 23 ◦C in C6D6
solution of both species reveal rapid rotation and rocking
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta 29.13◦
Absorption correction
9683 [Rint 0.0418]
100%
1
2
1
(h -Na → h → h -Nb) of the pyrazolide ligands to give
equivalent resonances for the t-Bu and Ph substituents. The
alkylidene protons of 1-dppz-Ph and 1-dtpz-Ph resonate at 13.35
and 13.10 ppm, respectively, and both show a JCH of 124 Hz. In
contrast using the Li or K salt of (3,5-dimethylpyrazole) only
leads to Mo(NAr)2(Me2pz)2 in approximately 50% yield as the
only isolable product, as determined by 1H NMR spectroscopy.
Semi-empirical
0.8432 and 0.8432
9683/1/526
1.054
R1 0.0311
wR2 0.0729
R1 0.0419
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2s(I)]
R indices (all data)
wR2 0.0795
0.518 and -0.583 e A
-3
˚
Largest diffraction peak and hole
Scheme 1 Synthesis of 1.
X-Ray quality crystals of 1-dppz-Ph were grown from a mixture
of pentane and toluene at -30 ◦C and a thermal ellipsoid plot
is shown in Fig. 1 (see Table 1 for crystal data). The complex
1-dppz-Ph contains
a
near linear imido (Mo(1)–N(1)–
C(11) 175.41◦) and the Mo(1)–C(1)–C(2) angle of 143.45◦ is
similar to other molybdenum imido alkylidenes.12 The dppz
ligands assume two different coordination modes. The dppz
Fig.
1
Solid state structure of compound 1-dppz-Ph. Selected
2
bond lengths (A) and angles (◦): Mo(1)–N(1) 1.7432(15),
Mo(1)–C(1) 1.8926(19), Mo(1)–N(2) 2.1026(16), Mo(1)–N(3) 2.1496(15),
Mo(1)–N(4) 2.0762(15); Mo(1)–N(1)–C(11) 175.41(14), Mo(1)–C(1)–
C(2) 143.45(14).
˚
ligand containing N(2) and N(3) is coordinated in a s,s-h
fashion according to the near symmetrical Mo–N bond lengths
˚
˚
of 2.1026(16) A and 2.1496(15) A. The other dppz ligand is
2
pseudo h coordinated with Mo(1)–N(4) equal to 2.0762(15)
A, which is similar to Mo –Npyrrolide bond lengths,12b and the
VI
˚
t-butanol, or diisopropylphenol in C6D6 solution (~40 mM) at 23
◦C cleanly produces Mo(NAr)(CHCMe2Ph)(OR¢)2 and two equiv-
alents of either 3,5-di-t-butylpyrazole or 3,5-diphenylpyrazole
2
˚
Mo(1)–N(5) distance equal to ~2.5 A. The reason for this h ,
h coordination which gives a formally 16 electron complex vs. a
h ,h configuration which would give a 18 electron complex is not
1
2
2
1
(Scheme 2), as determined by H NMR spectra of the reaction
known.
mixture. Formation of byproducts is not observable at the con-
centrations studied. Additionally, no monoalkoxide intermediates
were observed during 1H NMR monitoring of the reaction
mixture. All the in situ prepared bisalkoxide complexes were
shown to be competent catalysts for the ring-closing metathesis of
Reaction of bis-pyrazolide complexes with alcohols and phenols
Reaction of 1-dppz-Ph or 1-dtpz-Ph with two equivalents of either
t-butanol, trifluoro-t-butanol, hexafluoro-t-butanol, nonafluoro-
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 8547–8551 | 8549
©

signal appeared at 3608 cm-1 in agreement with the presence of
residual SiOH interacting with the hydrocarbyl ligands.14 Grafting
was also carried out by contacting 1-dppz-Ph and SiO2-(700) in C6H6,
and elemental analysis of the resulting solid shows the presence
of 1.62%wt Mo in the solid, which is also consistent with grafting
and a partial consumption of surface silanols. This corresponds to
0.17 mmol of grafted Mo and to a consumption of ca. 65% of the
surface SiOH (0.26 mmol g-1). Additionally, during grafting, only
0.2 equiv. of H(dppz) was liberated per grafted Mo. This shows
that grafting is probably not a simple exchange of one pyrazolide
by a siloxy ligand but most probably involves the formation of two
Scheme 2 Reactivity of 1 with alcohols.
1
grafted species 2-dppz-Ph and 3-dppz-Ph in 20 : 80 ratio. The H
diallyltosylamine or allylether to give 1-tosyl-2,5-dihydro-pyrrole
or 2,5-dihydrofuran, respectively, in full conversion. Reactions
with one equivalent of an alcohol or phenol were explored in
an effort to prepare monoalkoxide(aryloxide) monopyrazolide
which would be analogous to MAP catalysts.3 Reaction of 1-
dppz-Ph or 1-dtpz-Ph with one equivalent of HOCMex(CF3)y
(x 0, 1, 2, or 3; y 3 - x) in either toluene or pentane–
benzene between -30 - 23 ◦C, gave a complex mixture of
products from which no monoalkoxide monopyrazolide complex
could be obtained. Similarly, reaction of 1-dppz-Ph or 1-dtpz-
Ph with 2,6-diisopropylphenol or 2-bromo-4,6-ditertbutylphenol
gave mixtures of approximately 50 : 50 monoalkoxide monopy-
razolide:bisalkoxide. Attempts to selectively crystallize the
monophenoxide monopyrazolide product were unsuccessful.
Reaction of 1-dppz-Ph or 1-dtpz-Ph with [(R)-2¢-((di-tert-
butyl(phenyl)silyl)oxy)-1,1¢-naphthol3b (H(Br2Bitet-TBS)) failed
to yield any of Mo(NAr)(CH2CMe2Ph)(3,5-R2pz)(Br2Bitet-TBS)
although full consumption of the molybdenum starting material
was observed after heating the reaction to 60 ◦C for 18 h in C6D6
solution.
magic-angle spinning (MAS) (Figure S2a) of the resulting solid
displays 5 resolved signals at 12.7 ( CHMe2Ph), 7.8 (Csp2–H),
4.4 (C3N2H), and 1.8 (free OH, CHCMe2Ph and CHMe2).
Note that the proton of the alkylidene ligand at 12.7 ppm is
only observed as a very weak signal. The 13C CP MAS NMR
spectrum (ESI, Fig. S2b)† exhibits resonances assigned to the
aromatic carbons (152–128 ppm), and four more signals assigned
as follow: 107 (pyrazolide Csp2–H), 54 ( CHCMe2Ph), 28
(CHMe2Ph) and 22 ppm (CHMe2). As is typically the case for
the alkylidene signals of samples at natural 13C abundance (ca.
0.02 mmol of 13C g-1 of sample) no signal corresponding to the
alkylidene carbon resonance (Mo CHCMe2Ph) was observed.
We therefore investigated the grafting of the corresponding 13C
labelled neopentylidene complex, 1*-dppz-Me, 99% 13C labelled on
the a-carbon to Mo. In this case, the 13C CP-MAS NMR was very
informative by showing two new isotropic signals of 13C enriched
carbons at 70 and 298 ppm (ESI, Fig. S3b).† The signal at 298 ppm
is readily assigned to the alkylidene carbon of the neopentylidene
and is consistent with the formation of 2-dppz-Me, which results
from the protolysis of one of the pyrazolide ligand (Scheme 3). The
other signal at 70 ppm can however be attributed to the carbon of
a neopentyl ligand bound to the metal center, and we propose the
formation of 3-dppz-Me having two remaining dppz ligands, which
results from the addition of the silanol across the double bond of
the alkylidene ligand. Such species have already been observed
before, e.g. in the grafting on silica of bis-(2,5-dimethylpyrrolyl)
Mo complexes.4c Thus, combining available analytical data (in situ
IR, mass balance analysis and solid state NMR spectroscopy),
speaks for the formation of the desired surface complex 2-dppz-
R2 (R2 Me or Ph) as a minor surface species (20–30%), the
major species being 3-dppz-R2 (ca. 70–80%) where no alkylidene
is present (Scheme 3).
Reaction of bis-pyrazolide complexes with SiO2-(700)
In view of the high reactivity of complexes 1 with alcohols, we
studied their reactivity with the surface silanols of silica partially
dehydroxylated at 700 ◦C, SiO2-(700). with the hope to obtain
monosiloxy complexes 2 (Scheme 3).13 Monitoring the grafting
of 1-dppz-Ph on a pellet of SiO2-(700) by IR spectroscopy showed
that the peak associated with the surface silanols at 3747 cm-1
disappeared as new IR bands associated with the n(C–H) and d(C–
H) of hydrocarbyl ligands appeared in the 3000–2800 and 1500–
1350 cm-1 region, respectively (ESI, Fig. S1).† Moreover, a broader
Similar observations are found with the grafting of 1-dtpz-
Ph on SiO2-(700), i.e. partial consumption of OH groups and
formation of OH groups interacting with hydrocarbyl ligands
(ESI, Fig. S4),† a low Mo loading (1.58%wt) in agreement with
the partial consumption of the surface silanols (0.16 mmol of
Mo/g of silica, ca. 63%) and formation of 0.3 equiv. of H(dtpz)
1
per grafted Mo. The H MAS NMR and 13C CP-MAS NMR
spectra are also consistent with the grafting of molybdenum imido
alkylidene pyrazolide complexes (ESI, Fig. S5).† However, the
proton of the alkylidene ligand was only observed as a very
weak signal at 12.7 ppm in 1H NMR, and the alkylidene carbon
not observed under 13C CP-MAS NMR. Overall, the data are
consistent with the formation of two grafted surface species: 30%
of [( SiO)Mo(NAr)( CHCMe2Ph)(dtpz)] (2-dtpz-Ph) and 70%
of [( SiO)Mo(NAr)(CH2CMe2Ph)(dtpz)2] (3-dtpz-Ph).
Scheme 3 Reactivity of 1 with the surface silanols of SiO2-(700)
8550 | Dalton Trans., 2010, 39, 8547–8551
.
This journal is
The Royal Society of Chemistry 2010
©

Alkene metathesis
2 A. S. Hock, R. R. Schrock and A. H. Hoveyda, J. Am. Chem. Soc.,
2006, 128, 16373.
3 (a) R. Singh, R. R. Schrock, P. Mu¨ller and A. H. Hoveyda, J. Am.
Chem. Soc., 2007, 129, 12654; (b) S. J. Malcolmson, S. J. Meek, E. S.
Sattely, R. R. Schrock and A. H. Hoveyda, Nature, 2008, 456, 933; (c) I.
Ibrahem, M. Yu, R. R. Schrock and A. H. Hoveyda, J. Am. Chem. Soc.,
2009, 131, 3844–3845; (d) Y. J. Lee, R. R. Schrock and A. H. Hoveyda,
J. Am. Chem. Soc., 2009, 131, 10652–10661; (e) S. C. Marinescu, R. R.
Schrock, B. Li and A. H. Hoveyda, J. Am. Chem. Soc., 2009, 131, 58–
59; E. S. Sattely, S. J. Meek, S. J. Malcolmson, R. R. Schrock and A. H.
Hoveyda, J. Am. Chem. Soc., 2009, 131, 943–953.
4 (a) F. Blanc, C. Cope´ret, J. Thivolle-Cazat, J.-M. Basset, A. Lesage, L.
Emsley, A. Sinha and R. R. Schrock, Angew. Chem., Int. Ed., 2006, 45,
1216; (b) F. Blanc, J. Thivolle-Cazat, J.-M. Basset, C. Cope´ret, A. S.
Hock, Z. J. Tonzetich and R. R. Schrock, J. Am. Chem. Soc., 2007, 129,
1044; (c) F. Blanc, R. Berthoud, A. Salameh, J.-M. Basset, C. Cope´ret,
R. Singh and R. R. Schrock, J. Am. Chem. Soc., 2007, 129, 8434; (d) C.
Cope´ret, Dalton Trans., 2007, 5498.
5 (a) X. Solans-Monfort, E. Clot, C. Cope´ret and O. Eisenstein, J. Am.
Chem. Soc., 2005, 127, 14015; (b) A. Poater, X. Solans-Monfort, E.
Clot, C. Cope´ret and O. Eisenstein, J. Am. Chem. Soc., 2007, 129, 8207;
(c) X. Solans-Monfort, C. Cope´ret and O. Eisenstein, J. Am. Chem.
Soc., 2010, 132, 7750.
6 (a) D Ro¨ttger, G. Erker, M. Grehl and R. Fro¨lich, Organometallics,
1994, 13, 3897; (b) C. L. Dezelah IV, O. M. El-Kadri, M. J. Heeg and
C. H. Winter, J. Mater. Chem., 2004, 14, 3167; (c) O. M. El-Kadri, M. J.
Heeg and C. H. Winter, Dalton Trans., 2006, 1943.
The catalytic activity of 1-Ph/SiO2 in propene metathesis was
investigated (ESI, Fig. S6–9).† Firstly, contacting 1-dppz-Ph/SiO2,
with propene in a flow reactor (ca. 400 mL ¥ min-1; 4800 (propene)
¥ (Mo)-1 ¥ min-1), gives selectively ethene and 2-butenes (>
99.9%) with a very fast initial rate (1.7 s-1), and overall 25,300
TON. However, 1-dtpz-Ph/SiO2 is not as active, with an initial
rate of 0.5 s-1 and an overall TON of 3800 after 1500 min.
Considering the number of active sites (ca. 20%), the initial rate
and the overall number of turnover for 1-dppz-Ph/SiO2 can be
estimated to 8.5 s-1 and 127 000, respectively, so that 2-dppz-
Ph displays catalytic performances similar to those previously
reported using diphenylamido, pyrrolyl or 2,6-dimethylpyrrolyl
precursors.4 Note that in contrast 1-Ph is inactive in the metathesis
of 1-alkene at room temperature, which illustrates the importance
of the dissymmetry at the metal center to achieve high reactivity.5
4
Conclusions
Pyrazolide alkylidene Mo complexes can be readily
synthesized by reaction of lithium pyrazolide with
Mo(NAr)(CHCMe2Ph)(OTf)2(DME), when the pyrazolide
ligand is sufficiently bulky to avoid decomposition of the product.
These compounds readily react with alcohols to generate the
corresponding bis-alkoxide, but so far it has been impossible
to selectively generate the monopyrazolide alkoxide complexes.
In contrast, the use of SiO2-(700) could yield the correspond
monosiloxy complexes, albeit with a low selectivity because the
major product, a bis-pyrazolide siloxy complex, corresponds
to the addition of the SiO–H to the alkylidene ligand. It
is noteworthy however that these pyrazolide Mo alkylidene
complexes display similar catalytic performances in alkene
metathesis with the pyrrolyl systems, showing that catalytic
performances are not affected by the multiple binding modes of
pyrazolide ligands. Further studies are currently under way to
establish structure-reactivity relationship in alkene metathesis.4d
7 R. R. Schrock, J. S. Murdzek, G. C. Bazan, J. Robbins, M. DiMare and
M. O’Regan, J. Am. Chem. Soc., 1990, 112, 3875.
´
´
8 S. A. Corte´s-Llamas, R. Herna´ndez-Lamoneda, M. A. Vela´zquez-
´
Carmona, M. A. Mun˜oz-Herna´ndez and R. A. Toscano, Inorg. Chem.,
2006, 45, 286.
9 S. Beaini, G. B. Deacon, E. E. Delbridge, P. C. Junk, B. W. Skelton and
A. H. White, Eur. J. Inorg. Chem., 2008, 4586.
10 G. M. Sheldrick, Acta Crystallogr., Sect.A: Found. Crystallogr., 1990,
46, 467.
11 G. M. Sheldrick SHELXL-97, Universita¨t Go¨ttingen, Go¨ttingen,
Germany, 1997.
12 (a) U. Anders, O Nuyken, M. R. Buchmeiser and K. Wurst, Angew.
Chem., Int. Ed., 2002, 41, 4044; (b) K. M. Wampler, R. R. Schrock
and A. S. Hock, Organometallics, 2007, 26, 6674; (c) S. L. Aeilts, D. R.
Cefalo, P. J. Bonitatebus Jr., J. H. Houser, A. H. Hoveyda and R. R.
Schrock, Angew. Chem., Int. Ed., 2001, 40, 1452; (d) W. C. P. Tsang,
J. A. Jernelius, G. A. Cortez, G. S. Weatherhead, R. R. Schrock and
A. H. Hoveyda, J. Am. Chem. Soc., 2003, 125, 2591; (e) J. B. Alexander,
R. R. Schrock, W. M. Davis, K. C. Hultzsch, A. H. Hoveyda and J. H.
Houser, Organometallics, 2000, 19, 3700.
13 C. Cope´ret, M. Chabanas, R. Petroff Saint-Arroman and J.-M. Basset,
Angew. Chem., Int. Ed., 2003, 42, 156.
14 B. Rhers, A. Salameh, A. Baudouin, E. A. Quadrelli, M. Taoufik, C.
Cope´ret, F. Lefebvre, J.-M. Basset, X. Solans-Monfort, O. Eisenstein,
W. W. Lukens, L. P. H. Lopez, A. Sinha and R. R. Schrock,
Organometallics, 2006, 25, 3554.
References
1 A. Sinha, R. R. Schrock, P. Mueller and A. H. Hoveyda,
Organometallics, 2006, 25, 4621.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 8547–8551 | 8551
©